The exercise comes as US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov meet in London for last-minute talks before the Crimea referendum on Monday.

Kerry and Lavrov spoke by phone about the situation in Ukraine and Russia said the two had discussed "taking into account existing Russian and US proposals to normalise the atmosphere and provide for civil peace".

The United States has sent 12 F-16 fighter jets and 300 service personnel to Poland for a training exercise in response to the Ukraine crisis while Nato has dispatched Awacs [Airborne Warning and Control System] reconnaissance aircraft to Poland and Romania in order to monitor the Ukraine border after authorisation was granted by alliance ambassadors.